Adds a visible prop to Queue that triggers auto-scroll to the currently
playing track when the queue becomes visible. PlayerMobile passes the
drawer open state, PlayerDesktop passes whether a track exists.
Extract a generic VirtualList.svelte component that only renders
visible rows + overscan buffer using absolute positioning in a tall
sentinel div. Queue.svelte now uses VirtualList with 64px row height
(supporting 2-line titles) instead of rendering all items.
This reduces DOM nodes from 1000+ to ~20 for large queues.
